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Aviation History
1925
1925 - 0844.PDF
spscially worked on the metal cocque fuselage and had obtainedsome interesting results. The fuselage of the D.1C.1, with a covering 0-5 mm. thick, and with formers and stringerssuitably disposed, weighed 80 kgs., including engine mounting. In the case of wing construction the difficulties were muchgreater, owing to the much larger compressive loads. Although the author considered the Junkers type of tubalar multi-spar wing construction satisfactory, he thought the use of joints of the S.E.C.M. type for attaching the crinkled stripsof the internal trellis bracing to the tubular spars would greatly facilitate manufacture. In conclusion the author stated that the suspicion withwhich duralumin had hitherto been regarded in England was not justified, and that he thought those of our constructorswho had commenced to use duralumin wo a Id very soon become convinced that this was so. The Discussion Col. Belaiew then opened the discussion, and called uponLieut, de Vaisseau, Jacques Bos, Air Attache of the French Embassy in London, to say a few words. Speaking inFrench, Lieut. Bos said he was sure they all regretted the absence of M. Dewoitine, who was very sorry he could not bf-present. He was not, himself, a technician, but he had had sufficient experience of aviation on the service side to realisehow much they owed to the work of technicians, and he thought an interchange of ideas such as that provided by the readingof M. Dewoitine's paper before a British society would do a great deal of good. M. Gaston Montfort, President of the French Chamber ofCommerce in London, also speaking in French, said they had not reached their present position of metal constructionwithout troubles, and he referred to the material known as Alferium, a light aluminium alloy employed by the Schneiderfirm in their monoplane, and said that this had given some trouble at first, but these had now been overcome and themetal was now considered satisfactory. Major Nerincx, Military Attache to the Belgian Embassyin London, said he had looked forward to hearing a paper read in French in London. He expressed regrets that the BelgianAir Attache, the Chevalier Willy Coppens, could, not be present, but he was at present in residence in Paris. (WillyCoppens is now Belgian Air Attache to both London and Paris.—ED.) Major Davidson, American Military Attache, said he didnot feel competent to enter into a technical discussion. He was interested in the French factors of safety of 16. InAmerica they used a factor of 15, and they had been somewhat scared when, on stunt flights with machines equipped withaccelerometers, it was found that figures as high as 7g were reached in certain manoeuvres. Mr. J. D. North, Chief Engineer and Designer of Boultonand Paul's, said he was interested to observe in how many cases the experience of M. Dewoitine tallied with that ofBoulton and Paul. He would draw special attention to the paragraph commencing : " Enfin la creation du prototype-dependra," and the importance of technique. They had been addressing themselves to systematisation, and he did notfind the figure of 1 \ million francs excessive for tooling up. He gathered that this figure referred to special tools ratherthan to machine tools, as stated in the translation of the paper. He recalled that in England during the first few yearsof development work the use of duralumin for structural members was rigorously prohibited by the Air Ministry, andthat in this country we were thus obliged to solve all our problems in steel, even those problems to which steel was leastadapted. It was true that the British machines were day bombers of approximately 4,000 kgs. weight, but he could notagree that the problem was easier than with the fighter class. On the contrary, these twin-engined machines, with theirextensive and complicated equipment, offered, he believed, more difficult problems than the fighter. Incidentally,M. Dewoitine was in error in imagining that the fighter had not been produced in steel. The Siddeley " Siskin " of the R.A.F.was all steel. The specific properties of steel compared favourably withduralumin. The effective compression strength of steel was realised as a minimum at 65 tons/sq. in. (about 100 kgs./sq. mm.), while that of duralumin was, according to their experience, 15 tons/sq. in. (23 kgs./sq. mm.), which was equalto a steel only just over 40 tons/sq. in. (about 60 kgs./sq. mm.) in effective compression strength. It was true that occasion-ally higher figures were realised in duralumin, but similarly higher figures were obtained in steel, e.g., up to 80 tons/sq. in.(125 kgs. /sq. mm.). The specific elasticity of the two materials was substantially the same. Duralumin was particularlysusceptible to failure by reverse bending. Practically all DECEMBER 24, 1925 fatigue fractures in duralumin which he had seen had beendue to reverse bending being set up by vibration of the structure. Steel spars suitable for biplane structure were, in hisexperience, in most cases lighter than duralumin, since they got the full benefit of the greater specific strength. Thesespars were quickly and economically made, though the technique of their design and manufacture was complicated.M. Dewoitine would, he thought, be surprised to know that they hardened and tempered these spar sections after formingin lengths up to 75 ft., producing parts thereby of inter- changeable accuracy, i.e., within a few thousandths of aninch. He would also add that they obtained a shear stress of over 30 tons per square inch in* their rivets, which wereof alloy steel, and that the weight of rivets in steel parts compared quite well with duralumin parts. It was perhapsnot necessary to state that the design and material of these rivets had been carefully studied and they were specially made. Although they preferred to use steel for the primary struc-ture (e.g., the spars, fuselage longerons, interplane struts, engine mountings, &c.) it was their regular practice to useduralumin for the secondary structure (ribs, leading and trailing edges, &c). This combination gave, he believed,the most economical results from all standpoints. The difference in the value of E of the two materials, waseasily taken into account in stressing, and there were many more serious difficulties in stressing than that, particularlyin the multiply-redundant types of structure, such as multi- spars and skin-wings which the author favoured. M. De-woitine took his stand with the cantilever-metal-skin school. Mr. Xorth would agree that if this was to be the ultimateaim (and he was far from conceding that it was)—then Duralumin was indicated, as the heavy variations of stresswere more easily dealt with in the simpler forms permissible with duralumin. Steel did not lend itself to taper con-struction, and was, of course, quite impracticable as a cover- ing. He would say one thin<<, that the figures 2-15 (10-5kgs.) and 1 -64 (8 kgs.) lbs. per square foot, seemed heavy to one accustomed to biplane construction for fighters, anddimensional theory certainly did not indicate reduction of weight with increase of size, but the contrary. As the ordinary biplane type of construction was parti-cularly adaptable for steel spars, Mr. North was always anxious that his preference for that type should be sound,because as Mr. Harry Brearly once said, " the danger of research is that we tend to find what we are looking for."He could not help feeling that Dr. Rohrbach's advocacy of heavy loading for large seaplanes was unconsciously due tothe fact that without heavy loading his system of construc- tion would be impossible. He very much appreciated M. Dewoitine's paper, andthanked him for giving us the benefit of his experience. The two curses of aviation to-day were too much secrecy anfitoo little flying. Before the war, there was a very free interchange of ideas among those engaged in all branches ofaeronautical work, and he wished there was more of the same spirit today. Captain \V. H. Savers called attention to one very importantfactor in connection with metal aircraft construction. In the next war there would certainly not be enough sprucefor the needs of a country with a large Air Force. This point had been mentioned, but perhaps not stressed asmuch as it deserved, by the author of the paper, Whether Europe liked it or not, it would have, for this reason, to turnto all-metal construction. He pointed out that France, with her large supplies of aluminium, had naturally developedduralumin construction. In this country, we had the high- grade steels necessary, and had developed steel construction.In Germany, they used both to some extent. In the United States, where they owned almost all the world's aircrafttimber supply, it was interesting to note that relatively little metal construction had been carried out. Mr. J. S. L. Oswald, of the English Electric Co., referredto a new British flying-boat with metal hull, which had recently been successfully tested. This machine had come out aboutthe same weight as the wood machine, and he thought there could be no doubt that when the constructors had had moreexperience, they would be able to produce one for consider- ably lighter weight than the wood machine. Duraluminconstruction had several advantages. The number of man- hours was reduced, which was of importance, and the absenceof water soakage was avoided. Mr. C. V. Coates, M.A., of Birkbeck College, recalled hiswar-time experience at the Air Ministry. He was then a believer in metal construction and had pointed out thatmetal construction would pay if they could get an alloy with a strength of 14 tons per square inch. 844
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